This invention relates generally to ring laser gyroscopes, and more specifically, to a ring laser gyroscope which utilizes a single optical sensor in the generation of output signals typically generated by ring laser gyroscopes which incorporate multiple optical sensors.
A ring laser gyroscope utilizes interference of laser light within a ring optical cavity to detect changes in orientation and rate of turn. At least some known ring laser gyroscopes utilize two optical sensors, which provide signals to respective electronic circuits to generate ring laser gyroscope output signals. One such optical sensor is sometimes referred to as a laser intensity monitor (LIM) sensor, and the other optical sensor is sometimes referred to as a readout sensor.
The LIM sensor and associated electronic circuitry generate at least a LIM monitor signal, a residual path length control (PLC) modulation signal, and a residual single beam signal (SBS) which are utilized in the operation of the ring laser gyroscope. The readout sensor and its associated circuitry generate readout signals, which, in one known ring laser gyroscope, are ninety degrees out of phase from one another, representing an optical fringe pattern having a frequency and phase. The readout signals are utilized in the determination of changes in an orientation and a rate of turn, for example, of a flight platform in which the ring laser gyroscope is installed. More specifically, as the fringe pattern moves across the readout sensor, the readout sensor and associated circuitry produce a series of pulses, the number of pulses created represents an angle or orientation of the flight platform, and a rate at which the pulses are created is representative of a speed of rotation (e.g., a rotation rate) of the flight platform in which the ring laser gyroscope is mounted.
Such ring laser gyroscopes operate in one of a number of transverse electromagnetic (TEM) wave modes. A TEM wave mode describes a plane light wave (e.g., a laser beam) propagating through free space which has a particular intensity pattern of radiation measured in a plane perpendicular (transverse) to the propagation direction of the beam. One such wave mode is a TEM00 mode of oscillation. TEM00 is a mode illustrative of a laser projected as a single beam, and is sometimes referred to as a fundamental transverse mode and corresponds to a smooth distribution of light across the output of the laser.
As further described below, other modes (TEMs) may be utilized in ring laser gyroscopes. For example, a mode of TEM10 is best described as two bright spots oriented horizontally with a thin dark region between them (assuming the lasing plane is horizontal). Further, a mode of TEM01 is similar to TEM10 except that it is rotated by 90 degrees. Other TEMs (modes) are also possible, for example, a mode of TEM23 is illustrative of a laser having a rectangular array of spots, three horizontally and four vertically.
Drawbacks to the known two sensor ring laser gyroscopes include performance degradation due to mode competition (near the TEM00 mode), production cycle time, cost, sensor inventory due to a need to match a normal distribution of gyroscope fringe patterns to a corresponding grid pattern on the readout sensor, low readout signal (power), and gyroscope life due to the original signal strength degrading over time. Mode competition occurs when a higher-ordered transverse mode resonates with the operating mode. The frequency spacing between the two modes is a function of the cavity curvature, defined by the curvature of the mirrors that comprise the cavity. If the curvature changes, for example by stress induced by temperature changes, the frequency spacing can go to zero for certain transverse modes. This is particularly troublesome for transverse modes that are not discriminated against strongly by the internal body aperture.